1. Field of the Invention
This invention generally concerns the field of subsea production systems, but in particular is for an arrangement for breaking away subsea umbilicals in the event they are snagged. Still more particularly, this invention concerns an arrangement for shearing metallic or metal reinforced subsea umbilicals.
2. Description of the Prior Art
Offshore oil and gas fields are often developed using subsea production systems having the wells and related equipment installed directly on the seabed. A typical prior arrangement of a subsea production system 5 with a cluster of subsea wells 10 is illustrated in FIG. 1. In the system shown, a number of subsea wells are drilled in a cluster around a central subsea gathering manifold 12. Well jumper piping 11 couple the wells 10 to the subsea manifold 12. Subsea christmas trees installed on the wells control the flow of oil and/or gas from the wells. Production from each subsea tree is routed into the manifold 12 via jumper piping 11, and then it is transported back to shore via export pipelines 14 laid on the seabed.
Hydraulically actuated valves and chokes mounted on the subsea trees, and actuated valves mounted on the manifold 12, provide an arrangement to regulate and control the flow of produced fluids. Hydraulic fluids for operating the valves and chokes are delivered to the subsea production system via one or more main hydraulic supply umbilicals 16 laid on the seabed, as shown in FIG. 1. Distribution umbilicals 18 deliver fluids from the main umbilicals 16 to the individual subsea trees of wells 10 (and sometimes directly to the manifold 12 as well). Both the main umbilicals 16 and the distribution umbilicals 18 typically consist of several individual hoses or tubes enclosed within a protective sheathing. Umbilicals containing a dozen or more tubes are not uncommon. In addition to hydraulic fluids, the umbilicals may also deliver corrosion inhibitors, hydrate suppression chemicals, and/or other service fluids to the subsea system. In some cases, one or more tubes in the umbilical serve as vent lines for bleeding annulus pressure from the well casing and/or for depressurizing the manifold 12 and flowlines 14 (for hydrate control and/or remediation).
In the past, hydraulic umbilicals servicing subsea production systems have been constructed of thermoplastic hose. While thermoplastic hose was adequate for subsea applications in shallow to medium depth waters, it is not suitable for use in deep water where ambient hydrostatic pressure can be several thousand pounds per square inch. Some of the fluids contained within the umbilical tubes are significantly less dense than seawater, for example: methanol used for hydrate inhibition. In deep water the tubes containing low density fluids are subjected to a significant external pressure differential. Thermoplastic hose has limited resistance to collapse and is therefore unsuitable for such applications. Umbilical tubes used as xe2x80x9cventxe2x80x9d lines may also be subjected to high external collapse pressure during venting operations when internal pressure falls well below seawater ambient pressure. (Such conditions are typical during hydrate control operations.) Thermoplastic hoses are clearly not suitable for such venting operations, due to the collapse problem mentioned above. For these reasons, metallic tubes or metal reinforced hoses are replacing thermoplastic hoses in umbilicals serving subsea production systems in deep and ultradeep waters.
Although the new metallic tube umbilicals provide excellent collapse resistance, they could pose a serious threat to a subsea system unless adequate snag load protection is incorporated into the system design. With thermoplastic hose umbilicals, snag loads are a lesser concern because such hoses have relatively low tensile strengths. If a subsea umbilical were to be snagged, the thermoplastic hoses typically break away without damaging the attached subsea equipment. This is not the case for umbilicals constructed of metallic tubes, or metal reinforced hoses, because each tube has a tensile strength in the range of 15 kip or more. Some subsea equipment, particularly subsea Christmas trees, could be severely damaged if subjected to umbilical snag loads in excess of 20-40 kips. Since umbilicals containing 10 or more tubes are not uncommon, the total combined snag load which could be transmitted by the umbilical to the subsea equipment is clearly a concern. As a result, an effective and reliable load limiting break away device within the umbilical system is essential.
One approach, used in some prior art metal tube umbilicals, is to provide a sequential break away device based on staggered lengths of tubing. In the event of a snag, the individual tube lengths are sized so that they fail in tension, hopefully one at a time, as the individual tubes are stretched to their breaking point The shortest length of tubing should fail first when it reaches its ultimate stress, followed by the next longest tube, etc. In theory, this design should limit the maximum snag load transmitted to the subsea equipment However, this type of break away device has several disadvantages. First, a rather large physical space is typically needed to house the necessary mounting bullheads and the substantial lengths of staggered tubing required for proper operation. In addition, the high ductility and elongation of the metal tubing usually results in several tubes being loaded before the first tube has parted. Thus, if a snag occurs, several tubes may be transmitting load to the subsea equipment during the progressive break away, increasing the total snag load acting on the subsea equipment.
Some prior art thermoplastic hose umbilicals have been equipped with Guillotine type cutter devices which are designed to shear the entire umbilical assembly in the event of a snag. One typical guillotine-type umbilical shearing device is commercially available from Oceaneering Company of Tomball, Texas. The Oceaneering guillotine style xe2x80x9cweaklinkxe2x80x9d is normally installed on the unarmored umbilical jumper between the Umbilical Termination Assembly (UTA) and the Subsea Installation. The jumper is installed through the guillotine perpendicular to the jumper axis. Tensile loads are reacted through a chain assembly (shorter than the umbilical jumper) attached to the UTA and the subsea installation. Another guillotine weaklink device provides a large tapered guillotine blade to shear the multiple tubes spaced in a horizontal pattern through an opening facing the guillotine blade. Both devices use a cable or chain to actuate the guillotine cutter blade to sever the umbilical in the event of a snag. Intentional slack is provided in the umbilical to ensure that the cable or chain will become taut (and thereby actuate the guillotine blade to cut the umbilical) before excessive tensile loads are reacted into the attached subsea equipment. With the prior art Oceaneering, guillotine cuter device, the guillotine blade must shear several tubes within the umbilical simultaneously. This leads to a much higher break away load reaction into the attached subsea equipment than if the tubes were severed individually. The situation may also be similar for the second guillotine cutter device mentioned above if the tapered cutter blade causes individual tubes to xe2x80x9cbunch upxe2x80x9d due to side loading. Although these guillotine-type cutter devices work well on thermoplastic hose umbilicals (which are relatively easy to cut with reasonable loads), this type of break away device may not be applicable for use with metal tube or metal reinforced hose umbilicals due to excessive actuation load requirements.
A primary object of this invention is to provide an effective and reliable load limiting break away device for a subsea umbilical.
Another object is to provide a compact, reliable reduced force break away device for a metal tube subsea umbilical system.
Another object of the invention is to provide a breakaway device which not only limits the maximum snag load transmitted into attached subsea equipment, but also allows pre-selection of the order in which individual tubes of the umbilical are severed, thereby ensuring a more controlled break away function; for example with hydraulic lines powering fail-closed valves on subsea trees and manifold being severed first for enabling such valves to close (thereby shutting in the subsea wells) prior to severing lines which are (or could be) exposed to well bore pressure.
Another object of the invention is to provide a break away device which also incorporates an integral safety device that resists premature actuation and/or tube damage during normal installation operations.
The object identified above as well as other features and advantages of the invention are incorporated in a break away device which includes inner and outer bodies for severing individual tubes of a subsea umbilical in the event of a snag of the umbilical. The outer body has a longitudinal cavity through it with upper and lower slots through body walls which are spaced 180xc2x0 from each other. The outer body has a first connection arrangement at a first end. The upper slot has a blade secured adjacent to a second end of the outer body which faces inwardly in the slot toward the first end.
The inner body is positioned for telescopic movement within the cavity of the outer body with a first end of the inner body inserted into the cavity of the outer body with a second end extending outwardly from the second end of the outer body. The inner body has a second connection arrangement at the second end. The inner body is formed from a solid bar with a plurality of holes, one hole for each of the plurality of umbilical tubes. The holes have their axes aligned with upper and lower slots of the outer body.
A plurality of individual jumper tubes are connected between first end and second end umbilical termination devices. The jumper tubes extend through upper and lower slots of the outer body with only one tube provided for each hole of the inner body. A first tension resistant member, such as a cable is connected between the first connection arrangement of the outer body and the first umbilical termination device, and a second tension resistant member is connected between the second connection arrangement of the inner body and the second umbilical termination device. When large opposing forces act on the first and second umbilical termination devices, for example when a main subsea umbilical is snagged on the sea floor by an anchor of a vessel or the like, the inner body is pulled out of the cavity the outer body with the blade in the top slot severing jumper tubes and uncoupling the first and second umbilical termination devices.
The first and second termination devices may be umbilical termination heads of an xe2x80x9cin-linexe2x80x9d umbilical on the sea floor. Alternatively the termination devices may be an umbilical termination head connected to a main supply umbilical and an electro-hydraulic distribution module connected to subsea wells.